Spin Rate (SR)
Definition
A pitcher's Spin Rate represents the rate of spin on a baseball after it is released. It is measured in revolutions per minute.
The amount of spin on a pitch changes its trajectory. The same pitch thrown at the same Velocity will end up in a different place depending on how much it spins. (For instance, a fastball with a high Spin Rate appears to have a rising effect on the hitter, and it crosses the plate a few inches higher than a fastball of equal Velocity with a lower Spin Rate. Conversely, a lower Spin Rate on a changeup tends to create more movement.)
As more data have become available, most experts have agreed that fastballs and breaking balls are tougher to hit when they possess higher Spin Rates. In fact, some data suggest that Spin Rate correlates more closely than Velocity to swinging-strike percentage.
http://m.mlb.com/glossary/statcast/spin-rate